The present invention relates to a heat pipe and method that incorporates thermal storage within the heat pipe and eliminates problems associated with incongruent melting, poor thermal conductivity, and the like. More particularly, the present invention is directed to a heat pipe which uses vapor-solid thermal storage of the excess heat pipe working fluid vapor within the heat pipe itself. Because the thermal storage is integrated into the heat pipe and uses the heat pipe working fluid, the thermal storage system is compact and lightweight.
A basic problem with all satellites is the problem of heat rejection. The problem is compounded in low earth orbit satellites where the effective space temperature for radiation heat transfer is quite high (typically 227K), thus requiring in relatively large thermal radiators. For all satellites, however, the problem is complicated by the lack of available surface area and/or the necessary radiation size of the radiators.
One solution used in high power satellites has been the use of a heat pump to elevate the radiation rejection temperature and thereby reduce the area of the thermal radiator. Although this approach works for all size satellites, the mass and area reductions are greater for high-power satellites. Heat pumps may also have some beneficial applications in smaller satellites, but typically these small satellites have accomplished their heat rejection requirements with a passive heat rejection system. As a matter of fact, the use of an active system is perceived as a major drawback in small satellites.
Cyclic thermal loads on the spacecraft thermal control system require that the thermal control system be sized for the maximum thermal load or that thermal storage to average the thermal load uniformly over the entire orbital cycle be utilized. Spacecraft applications have other restrictions, which include minimal system mass and system volume, and long-term reliability. Although it is not desirable to increase the size/capacity of the thermal control system, to accommodate the peak thermal load, up to now this has been the only effective technique available, especially in very small satellites in which the thermal storage structure and control system may be a significant fraction of the entire thermal storage device.
Current thermal storage devices also suffer from long-term performance problems. For example, phase change materials exhibit incongruent melting, poor thermal conductivity in the solid phase, and problems with resolidification. Metal hydrides are heavy and they compact due to fragmentation on repeated cycling. Sensible heat storage is too large and heavy.
Spacecraft applications, which have cyclic thermal loads that must be rejected to space through a radiator system, thus present a major problem. The typical spacecraft system is very mass-and-radiator-area sensitive and, at the same time, suffers from large thermal spikes which are many times the base load. Currently, no thermal storage system has provided a reliable, repeatable, compact storage system for small satellites.
When heat pipe transport capacity is insufficient (i.e., during increased evaporator cooling demands or with reduced condenser rejection capability), the heat pipe temperature and pressure normally rise due to the increased generation of vapor in the evaporator or the reduced condensation of vapor in the condenser. This excess vapor needs to be absorbed or swept away in some manner, or the pressure and temperature in the heat pipe will continue to rise, resulting in undesired increased heat pipe operating temperatures which will damage the equipment being cooled or at least severely decease their service life.
It is an object of the present invention to solve thermal problems associated with low-power, small satellites whose duty cycle is such that thermal storage reduces radiator requirements in light of the fact that the thermal rejection requirements are currently not uniformly spread over the entire orbital time.
It is yet another object of the present invention to provide a thermal storage heat pump and method for small satellites utilizing a passive, thermal storage heat pipe, i.e., a heat pipe that behaves as an ordinary heat pipe but can also store a significant amount of energy within the pipe in those instances when the heat load exceeds the heat rejection capability of the thermal radiators.
It is still a further object of the present invention to provide a heat pump which has other applications, including the addition of thermal storage within a hardened radiator assembly, by using the thermal storage heat pipes instead of conventional heat pipes to distribute the energy to individual radiator sections.
The foregoing objects have been achieved in accordance with the present invention by using a heat pipe thermal method and system with an adsorption chamber connected to the vapor space of the heat pipe. This chamber contains an absorbent for the heat pipe working fluid that can adsorb the heat pipe vapor.
In the present invention, a slight increase in pressure or temperature, the actual amount being a system variable, will cause a pressure- or temperature-actuated valve to open, allowing the vapor to flow into an adiabatic adsorption chamber where the vapor is adsorbed by the adsorbent material. The heat pipe continues to cool because the evaporator continues to evaporate liquid. The resulting vapor flows into this chamber to be adsorbed. The liquid to be evaporated is supplied from the liquid located in liquid artery and condenser sections of the heat pipe. The process continues until the heat pipe is depleted of liquid or the vapor adsorption chamber is saturated. The heat pipe can be configured so that these two events occur simultaneously, or the adsorption chamber can saturate first, allowing the thermal storage heat pipe to continue to function as an ordinary heat pipe after the adsorption chamber is saturated.
The adsorption chamber is later discharged when the condenser capacity exceeds the evaporator load. This thermal storage heat pipe thus has only one moving part, namely a pressure or temperature-actuated valve configured, for example, as a spring-loaded pressure or bimetallic thermal valve.
Inasmuch as adequate data is not available for the rate at which working fluid is adsorbed on an adiabatic adsorption bed, simple adsorption experiments verify that the adsorption and desorption for the present invention is rapid enough for spacecraft thermal control applications. These experiments were performed for the adsorption and desorption of methanol on a molecular sieve. FIG. 1 illustrates how rapidly the working fluid is adsorbed or desorbed from the adsorbent materia. In the adsorption experiment, the refrigerant i.e., methanol, was added to one cylinder. The system was evacuated and the valve between the refrigerant and the adsorbent opened. The methanol vapor flowed from the first cylinder, which simulated the heat pipe vapor core, and was adsorbed on the molecular sieves in the other cylinder. The temperature, weight, and pressure were monitored. The quantity of working fluid adsorbed appears consistent with the available commercial sieve data.
A number of different refrigerant working fluids are contemplated along with a number of adsorbent materials to provide significant thermal storage capacity within a heat pipe. One exemplary system uses water as the refrigerant and a molecular sieve as the adsorbent material. A significant thermal storage capability is thereby achieved. The method of the present invention can be used, however, with any heat pipe working fluid, except possibly the liquid metal heat pipes.